The wavelength (frequency) of light output from a light source such as a semiconductor laser changes, depending on the temperature, driving electric current, and so forth of the light source. As a wavelength control method for stabilizing (locking) the output light of a light source at a predetermined wavelength, there is known for example a method where a cyclic filter (for example, an etalon filter or the like) having a characteristic in which transmittance with respect to wavelength changes at a constant cycle, is utilized as a control reference. In this wavelength control method, there is defined a point that serves as a wavelength control target that conforms to the transmission wavelength characteristic of the cyclic filter (hereunder, referred to as “a wavelength locking point”). Then the light output from a light source is given to the cyclic filter, a transmittance of the cyclic filter is found based on the result of monitoring the power of the transmitted light, and a feedback control of the temperature of the light source is performed, so that the transmittance becomes a value that corresponds to the above wavelength locking point. As for the cyclic filter that serves as the above control reference, in order to suppress variation in the transmission wavelength characteristic caused by temperature changes, there may be used materials having a low level of temperature dependency, or there may be performed a control that maintains the filter temperature at a constant temperature, using a temperature monitor.
In such a wavelength control method as above, in those cases where the wavelength locking point is set on a peak portion in the transmission wavelength characteristic of the cyclic filter (in the vicinity of the maximal peak of transmittance) or on a valley portion (in the vicinity of the minimal peak of transmittance), variation in the transmitted light power with respect to variation in wavelength becomes smaller. Therefore it becomes difficult to ensure a sufficient level of transmittance monitoring sensitivity. Consequently, the wavelength locking point is normally set on a slope portion positioned intermediately between the peak portion and the valley portion. Specifically, for example as illustrated in FIG. 1, wavelength locking points A, B, C, and so on are defined in the approximate center of the right-up slope portions which rise from the valley to the peak in the transmission wavelength characteristic of the cyclic filter. Moreover, although here omitted in the diagram, wavelength locking points may also be defined on the right-down slope portions which fall from the peak to the valley, or on both of the slope portions which rise from the valley to the peak and fall from the peak to the valley.
In the setting of such wavelength locking points, intervals of the output wavelength of the light source that can be controlled by the above wavelength control method, are dependent on the cycle of the cyclic filter. In the example of FIG. 1, the free spectrum range (FSR) of the cyclic filter is 50 GHz. Therefore intervals of the output wavelength of the light source that can be controlled become 50 GHz in a case where one of the right-up slope portion and the right-down slope portion is used, and they become 25 GHz in a case where both of the slope portions are used. Here FSR represents frequency intervals between adjacent transmission peaks.
Incidentally, in a wavelength division multiplexing (WDM) type optical communication system, it is required in recent years that intervals of the wavelength (frequency) of a plurality of optical signals constituting a WDM light can be variably set on the user side (this may also be called multigrid). In a case where, in response to this requirement, a wavelength control of each optical signal is performed with an application of the above wavelength control method, there is a problem in that the level of control precision may possibly be reduced for the wavelength intervals that do not correspond to the cycle of the cyclic filter used as a control reference. For example, assuming a case where the aforementioned cyclic filter of 50 GHz FSR is used, and wavelength control that corresponds to a grid of 37.5 GHz intervals is to be performed, then as illustrated in FIG. 2, a high level of control precision is realized at wavelength locking points A and C set on the slope portions of the transmission wavelength characteristic of the cyclic filter, while a drop in the level of control precision at wavelength locking points B and D set on the peak portions and the valley portions cannot be avoided.
In relation to the above problem, as for the wavelength control method that uses a cyclic filter, there has been proposed a method in which a plurality of cyclic filters having different cyclic characteristics are combined to thereby perform wavelength control. For example, International Publication Pamphlet No. WO 2004/068660 discloses a technique in which a first etalon having a relatively long cycle and a second etalon having a relatively short cycle are combined. Then after having locked the output wavelength of a laser diode (LD) with the first etalon, the temperature of the LD chip at this time is changed by a predetermined amount to shift the wavelength into a target wavelength pull-in range of the second etalon. Furthermore the output wavelength of the LD is locked with the second etalon, and thereby the wavelength of the LD can be locked to the target wavelength even for narrow wavelength intervals. Such a technique for controlling wavelength, in which cyclic filters of different transmission wavelength characteristics are combined, may become one of the effective methods that enable multigrid capability.
However, in order to realize wavelength control that is capable of handling multigrid with a combination of cyclic filters of different transmission wavelength characteristics, it is necessary to prepare a number of cyclic filters capable of handling all assumed wavelength intervals, and there is a problem in that the configuration becomes complex.
Specifically, there is described a configuration in a case where wavelength intervals as multigrid, namely 50 GHz, 37.5 GHz, 33.3 GHz, and 25 GHz are assumed, and wavelength control capable of handling all of the wavelengths is realized. In this case, assuming a cyclic filter having a 50 GHz FSR serving as a reference, then for wavelength intervals of 50 GHz, as illustrated in the first row of FIG. 3, wavelength locking points A, B, and C may be set on the slope portions which rise from the valley to the peak in the transmission wavelength characteristic of the cyclic filter (or on the slope portions which fall from the peak to the valley), to thereby perform wavelength control. Moreover, for wavelength intervals of 25 GHz, the wavelength intervals become ½ of 50 GHz. Therefore as illustrated in the second row of FIG. 3, it is possible to perform wavelength control by setting wavelength locking points A, A′, B, B′, C, and C′ on both of the slope portions which rise from the valley to the peak and fall from the peak to the valley in the transmission wavelength characteristic of the cyclic filter.
On the other hand, for wavelength intervals of 37.5 GHz, the wavelength intervals become ¾ of 50 GHz. Therefore if the above reference cyclic filter (FSR=50 GHz) is used, wavelength locking points are to be set on the peak portions and valley portions in the transmission wavelength characteristic (refer to FIG. 2). If the wavelength locking points are set on the peak portions and valley portions as described above, the level of wavelength control precision is reduced. Therefore as illustrated in the third row of FIG. 3, it is consequently necessary to prepare a cyclic filter having a 37.5 GHz FSR, with a cycle that differs from that of the reference cyclic filter, and set wavelength locking points D, E, and F on the slope portions which rise from the valley to the peak (or on the slope portions which fall from the peak to the valley) in the transmission wavelength characteristic, to thereby perform wavelength control.
Moreover, for wavelength intervals of 33.3 GHz, the wavelength intervals become approximately ⅔ of 50 GHz, and consequently, wavelength locking points are set in the vicinity of the peak portions and valley portions in the transmission wavelength characteristic of the reference cyclic filter. Therefore, as illustrated in the fourth row of FIG. 3, it is necessary to prepare a cyclic filter having a 33.3 GHz FSR, which further differs from those of the respective cyclic filters of 50 GHz FSR and 37.5 GHz FSR, and set wavelength locking points G, H, and I on the slope portions which rise from the valley to the peak (or on the slope portions which fall from the peak to the valley) in the transmission wavelength characteristic, to thereby perform wavelength control.
Consequently, in order to realize wavelength control capable of handling multigrid of wavelength intervals of 50 GHz, 37.5 GHz, 33.3 GHz, and 25 GHz, it is necessary to prepare three types of cyclic filters having different cycles, provide a monitoring system for each of the cyclic filters, and perform control of the output wavelength of the light source while switching the monitoring systems according to wavelength interval settings, and this will cause the configuration to become more complex. If the conditions of the wavelength intervals increase, the number of required cyclic filters may also increase, and consequently an even more complex configuration will be necessary.